Action Points

Note that this observational study linking urinary phthalate levels in mothers and children found associations between these levels and thyroid hormones in some individuals.

Be aware that the effects on thyroid function were modest, with all variation falling within the normal range.

Preschool age girls exposed to phthalates, a class of endocrine disrupting chemicals found in plastics and shampoos, showed signs of depressed thyroid function, according to a study of inner-city mothers and children.

Levels of the active thyroid hormone free thyroxin (FT4) were lower in 3-year-old girls with higher exposure to mono-n-butyl phthalate, mono-isobutyl phthalate, monoethyl phthalate, and other phthalate metabolites, reported senior investigator Pam Factor-Litvak, PhD, of the Mailman School of Public Health at Columbia University in New York City, and colleagues.

"Investigators have reported that disturbances in thyroid function are more prevalent among women. Thus, girls may be more vulnerable to the effect of thyroid disrupting chemicals even in early childhood," Factor-Litvak and colleagues said in the journal Environment International.

The study found no association between phthalates and FT4 levels in three-year-old boys, and no association between phthalates and thyroid stimulating hormone (TSH) in preschool age children of either sex.

However, phthalate exposure in mothers during late pregnancy was associated with increased, rather than decreased, levels of FT4 in both girls and boys. This result suggests that phthalates may affect thyroid function differently depending on the age of exposure, the investigators said.

"The thyroid acts as the master controller of brain development," Factor-Litvak said in a statement. "Thyroid hormones set the schedule, and if the timing is out of synch, there may be later consequences in the brain. The thyroid disruptions we see in this study, although they fall within the normal range, could explain some of the cognitive problems we see in children exposed to phthalates, and we are currently investigating that. As we know from lead, even small exposures can make a big difference."

Leonardo Trasande, MD, of the New York University School of Medicine in New York City, agreed that the results were cause for concern even though FT4 levels were still within the normal range. "Decreases in FT4 in pregnancy have been associated with worse cognitive outcomes in children as well as ADHD and autism. Given that brain development remains rapid after birth during early childhood, decreases in thyroid levels are likely to be important with implications for later neurocognitive outcomes," he said in an email to MedPage Today.

"Parents with young children should avoid using products containing phthalates such as shampoos, nail polish, and vinyl flooring," Factor-Litvak said.

Trasande, who was not involved in the study, added that phthalates are also found in plastics. "There are safe and simple steps to limit phthalate exposures," he explained. "These include: avoiding microwaving plastic food containers; avoiding drinking from plastic bottles with the recycling numbers 3, 6, and 7; handwashing (rather than machine dishwashing) plastic food containers; and disposing of them when they are obviously etched or scratched."

The study included 181 mother-child pairs enrolled in the Columbia Center for Children's Environmental Health cohort. This cohort was restricted to non smoking women 18–35 years old who self-identified as either African American or Dominican. They lived in northern Manhattan or the South Bronx in New York City for at least 1 year before pregnancy. Women were excluded if they used illicit drugs, had diabetes, hypertension, HIV, or had their first prenatal visit after the 20th week of pregnancy.

The investigators collected spot urine samples from women during their third trimester of pregnancy and from the children at age 3. They analyzed these for the presence of phthalate metabolites. The investigators also measured serum FT4 and TSH levels in the children at age 3. They used linear regression models to explore associations between phthalate exposure and thyroid function.

The mean concentration of FT4 in the children was 1.42 ng/dL (range 1.02-2.24) and the mean concentration of TSH was 2.62 uIU/mL (range 0.61-11.67).

Limitations of the study included the small sample size and that the results may not be generalizable to mothers and children of other races and ethnicities, the investigators said. In addition, they noted the study measured phthalate exposure at only two time points, and that other time points might be more important predictors of thyroid function.

"Our results contribute to a growing body of literature suggesting that early life exposure to some phthalates may affect endocrine function in children," the investigators concluded.

"Going forward, it's important to learn what phthalates do to harm children, as well as the route by which this harm is inflicted," Factor-Litvak said. "Our overarching goal is to protect the health of future generations."

The study was funded by the National Institute of Environmental Health Sciences and the U.S. Environmental Protection Agency.

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